The present invention relates to the x-ray tube art. It finds particular application in conjunction with high power x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It will be appreciated, however, that the invention will also have other applications.
Typically, a high power x-ray tube includes a cathode filament through which a current of about 5 amps is passed at a voltage sufficient to provide about 75 watts of power. This current heats the filament sufficiently that it is caused to emit a cloud of electrons, i.e. thermionic emission. A high potential on the order of 100 kV is applied between the cathode and the anode. This potential causes the electrons to flow between the cathode and the anode through the evacuated region in the interior of the envelope. Generally, this electron beam or current is on the order of 10-500 mA. The electron beam impinges on the anode generating x-rays and producing extreme heating as a byproduct. In high energy x-ray tubes, the anode is rotated at high speeds such that the electron beam does not dwell on only a small area of the anode causing thermal deformation. Each spot on the anode which is heated by the electron beam cools substantially during one rotation of the anode before it is again heated by the electron beam. Larger diameter anodes have a larger circumference, hence provide greater thermal loading. In most conventional rotating anode x-ray tubes, the envelope and the cathode remain stationary while the anode rotates inside the envelope. The anodes dissipate heat by thermal radiation across the evacuated interior of the envelope. As more energy is put into the anode of larger tubes to produce more x-rays, the inefficiency of thermal radiation limits cooling, hence x-ray production.
In order to avoid this heat transfer difficulty, high power x-ray tubes have been proposed in which the anode and vacuum envelope rotate, while the cathode filament inside the envelope remains stationary. This configuration permits a heat transfer fluid to be circulated in direct contact with the anode to remove heat more efficiently. See for example, U.S. Pat. Nos. 4,788,705 and 4,878,235. One of the difficulties with this configuration is providing electrical energy to the stationary cathode within the rotating vacuum envelope. Conveying 5 amps of power into an evacuated envelope without degrading the vacuum can be achieved by using an air core coil or an air core transformer as illustrated by the above-referenced patents. One drawback of the air core coil or transformer configurations is that the filament current cannot be measured directly. Only the primary current of the transformer can be measured and the primary current is a complex function of core temperature, flux density, air gap length, and the like. Second, any vibration of the cathode structure induces changes in the magnetic flux linking the external primary and the internal secondary. These vibration induced changes in the flux linkage cause corresponding variations in the filament current, leading to erratic filament emission. A third drawback to these patents is that the air core coil or transformer operates at about 13.56 MHz which corresponds a skin depth in copper of about 0.024 mm. Because the electrical current is constrained to such a shallow skin depth, problems arise in the design of the low-resistance leads to the filament, as well as to localized hot spots on the filament itself.
The present invention provides a new and improved technique for transferring electrical power to the filament of an x-ray tube in which there is relative rotational movement between the envelope and the cathode.